I've heard GL-5 is bad news for transmissions that specifically call for GL-4, supposedly they corrode the bronze and copper inside, synchros.... What about using it in a transfer case?

Transfer cases use various different fluids. If yours requires a GL5, you have nothing to worry about.

You need heat and moisture to cause issues. I've used GL5 in manual trannies with no problems. But, I practice more frequent change intervals than most people. And, I don't drive through rivers. I believe that the ANTI-GL5 argument in a manual trans is way overhyped.

I use GL5 Elf Transaxle which is safe for soft metal on my Puch T case as well as diffs, I also the same fluid on my antique 69 Nissan Patrol diff and tranny with great results, I get far less metal shavings when draining oil than I used to when using 80W-90 GL4 oil, I truly like the extended protection and cushion offered by the 85W-140 GL5 oil, especially in summers as well as for severe off roading.